eponychium (ep-o-nik′e-um) [TA]
1. The thin, condensed, eleidin-rich layer of epidermis that precedes and initially covers the nail plate in the embryo. It normally degenerates by the eighth month except at the nail base where it remains as the cuticle of the nail. 2. [NA] The corneal layer of epidermis overlapping and in direct contact with the nail root proximally or the sides of the nail plate laterally, forming the undersurface of the nail wall or nail folds. SYN: hidden nail skin, perionychium. 3. The thin skin adherent to the nail at its proximal portion. SYN: epionychium. [G. epi, upon, + onyx (onych-), nail]

eponym (ep′o-nim)
The name of a disease, structure, operation, or procedure, usually derived from the name of the person who discovered or described it first. [G. eponymos, named after]

eponymic (ep-o-nim′ik)
1. Relating to an eponym. 2. An eponym.

epoophoron (ep′o-of′o-ron)
A collection of rudimentary tubules in the mesosalpinx between the ovary and the uterine tube; composed of two portions, the longitudinal duct of epoöphoron and the transverse ductules of epoöphoron, they are the vestiges of tubules of the middle portion of the mesonephros and the homologue of the aberrant ductules and proximal duct of epididymis in the male. SYN: corpus pampiniforme, organ of Rosenmüller, pampiniform body. [epi- + G. oophoros, egg-bearing]

epoxy (e-pok′se)
Chemical term describing an oxygen atom bound to two linked carbon atoms Generally, any cyclic ether, but commonly applied to a 3-membered ring; specifically, a three-membered ring is an oxirane, a four-membered ring is an oxetane, a five-membered ring is an oxolane, and a six-membered ring is an oxane; oxiranes are commonly produced from peracids acting on alkenes. Epoxys are important chemical intermediates and the basis of e. resins (polymers) formed from e. monomers.

2,3-epoxysqualene (e-pok′se-skwa′len)
An oxirane derivative of squalene; a precursor to all of the steroids.

equation (e-kwa′zhun)
A statement expressing the equality of two things, usually with the use of mathematical or chemical symbols. [L. aequare, to make equal]
alveolar gas e. the e. defining the steady state relation of the alveolar oxygen pressure to the barometric pressure, inspired gas composition, alveolar carbon dioxide pressure, and respiratory exchange ratio; the e. is used in various forms depending upon which simplifying assumptions are acceptable for different applications.
Arrhenius e. an e. relating chemical reaction rate (k) to the absolute temperature (T) by the e.: d(ln k)/dT) = ΔEa/RT2 where Ea is the activation energy and R is the universal gas constant.
Bohr e. an e. to calculate the respiratory dead space from the fact that gas expired from the lungs is a mixture of gas from the dead space and gas from the alveoli, i.e., the dead space volume divided by the tidal volume equals the difference between alveolar and mixed expired gas composition, divided by the difference between alveolar and inspired gas composition; gas composition can be expressed in any consistent units of concentration or partial pressure of oxygen or carbon dioxide.
chemical e. an e. on one side of which are the reactants and on the other side of which are the products of a chemical reaction; the two halves may be separated by an equals sign or by arrows.
constant field e. SYN: Goldman e..
Einthoven e. SYN: Einthoven law.
Gay-Lussac e. the overall chemical e. for alcoholic fermentation; C6H12O6 = 2CO2 + 2CH3CH2OH.
Gibbs-Helmholtz e. 1. an e. expressing the relationship in a galvanic cell between the chemical energy transformed and the maximal electromotive force obtainable. 2. ΔG = ΔH = T[&pardef;ΔG/&pardef;T] P, where ΔG is the change in Gibbs free energy, ΔH is the change in enthalpy, T is the absolute temperature, and P is the pressure.
Goldman e. an e. derived to predict membrane potentials in terms of the membrane's permeability to ions and their concentrations on either side. SYN: constant field e., Goldman-Hodgkin-Katz e., GHK e..
Goldman-Hodgkin-Katz e., GHK e. SYN: Goldman e..
Henderson-Hasselbalch e. a formula relating the pH value of a solution to the pKa value of the acid in the solution and the ratio of the acid and the conjugate base concentrations: pH = pKa + log([A−]/[HA]), where [A−] is the concentration of the conjugate base and [HA] is the concentration of the protonated acid. For the bicarbonate buffer system in blood, pH = pK′ + log([HCO3−]/[CO2]. The value of pK′ for blood plasma is 6.10 and includes the first dissociation constant of H2CO3, the relation between [H2CO3] and [CO2] and other corrections. The partial pressure of CO2 multiplied by its solubility in plasma at 38°C (0.0301 mM/mm Hg) is commonly substituted for [CO2]; e.g., when the plasma bicarbonate concentration is 24 mEq/L and the PCO2 is 40 mm Hg, the pH value is 6.10 + log(24/0.0301 × 40) = 7.40.
Henri-Michaelis-Menten e. SYN: Michaelis-Menten e..
Hill e. the e. y(1 − y) = [S]n/Kd, where y is the fractional degree of saturation, [S] is the binding ligand concentration, n is the Hill coefficient, and Kd is the dissociation constant for the ligand. The Hill coefficient is a measure of the cooperativity of the protein; the larger the value, the higher the cooperativity. This coefficient cannot be higher than the number of binding sites. For the oxygen binding curve of hemoglobin, an association constant, Ka, is used and the e. becomes y/(1 − y) = Ka[S]n. For human hemoglobin, n = 2.5. Cf.:Hill plot.
Hüfner e. an e. expressing the relationship between myoglobin dissociation and oxygen partial pressure: ([MBO2]/[Mb]) = (K × pO2).
Lineweaver-Burk e. a rearrangement of the Michaelis-Menten e., 1/v = 1/Vmax + (Km/Vmax)(1/[S]), where v is the velocity of the reaction, Vmax is the maximum velocity, Km is the Michaelis constant; and [S] is the substrate concentration. Cf.:double-reciprocal plot.
Michaelis-Menten e. an initial-rate e. for a single-substrate non-cooperative enzyme-catalyzed reaction relating the initial velocity to the initial substrate concentration; v = Vmax [S]/(Km + [S]), where v is the initial velocity of the reaction, Vmax is the maximum velocity, [S] is the initial substrate concentration, and Km is the Michaelis constant. Similar equations can be derived for conditions in which the product is present and for multisubstrate enzymes. SYN: Henri-Michaelis-Menten e..
Nernst e. the e. relating the equilibrium potential of electrodes to ion concentrations; the e. relating the electrical potential and concentration gradient of an ion across a permeable membrane at equilibrium: E = [RT / nF] [ln (C1/C 2)], where E = potential, R = absolute gas constant, T = absolute temperature, n = valence, F = the Faraday, ln = the natural logarithm, and C1 and C2 are the ion concentrations on the two sides; in nonideal solutions, concentration should be replaced by activity. SEE ALSO: activity (2) .
personal e. a slight error in judgment, perceptual response, or action peculiar to the individual and so constant that it is usually possible to allow for it in accepting the person's statements or conclusions, thus arriving at approximate exactness; observed in persons whose work involves readings of events in time, such as navigators and air traffic controllers.
rate e. a mathematical expression for a chemical, radiochemical, or enzyme-catalyzed reaction.
Rayleigh e. a ratio of red to green required by each observer to match spectral yellow. SYN: Rayleigh test.
Svedberg e. See sedimentation constant.
van't Hoff e. 1. e. for osmotic pressure of dilute solutions. See van't Hoff law. 2. for any reaction, d(ln Keq/d(1/T) equals −ΔH/R where Keq is the equilibrium constant, T the absolute temperature, R the universal gas constant, and ΔH the change in enthalpy; thus, plotting ln Keq vs. 1/T allows the determination of ΔH.

equator (e-kwa′ter) [TA]
A line encircling a globular body, equidistant at all points from the two poles; the periphery of a plane cutting a sphere at the midpoint of, and at right angles to, its axis. [Mediev. L. aequator, fr. L. aequo, to make equal]
e. bulbi oculi [TA] SYN: e. of eyeball.
e. of eyeball [TA] an imaginary line encircling the globe of the eye equidistant from the anterior and posterior poles. SYN: e. bulbi oculi [TA] .
e. of lens [TA] the periphery of the lens lying between the two layers of the ciliary zonule. SYN: e. lentis [TA] .
e. lentis [TA] SYN: e. of lens.

equatorial (e-kwa-to′re-al)
Situated, like the earth's equator, equidistant from each end.

equilibration (e′kwi-li-bra′shun, e-kwil-i-)
1. The act of maintaining an equilibrium or balance. 2. The act of exposing a liquid, e.g., blood or plasma, to a gas at a certain partial pressure until the partial pressures of the gas within and without the liquid are equal. 3. In dentistry, modification of occlusal forms of the teeth by grinding, with the intent of equalizing occlusal stress, producing simultaneous occlusal contacts, or harmonizing cuspal relations. 4. In chromatography, the saturation of the stationary phase with the vapor of the elution solvent to be used.

equilibrium (e-kwi-lib′re-um)
1. The condition of being evenly balanced; a state of repose between two or more antagonistic forces that exactly counteract each other. 2. In chemistry, a state of apparent repose created by two reactions proceeding in opposite directions at equal speed; in chemical equations, sometimes indicated by two opposing arrows (⇆) instead of the equal sign. SYN: dynamic e.. SEE ALSO: e. constant. [L. aequilibrium, a horizontal position, fr. aequus, equal, + libra, a balance]
acid-base e. SYN: acid-base balance.
Donnan e. when a semipermeable membrane or its equivalent ( e.g., a solid ion fexchanger) separates a nondiffusible substance, such as protein, from diffusible substances, the diffusible anions and cations are distributed on the two sides of the membrane so that 1) the products of their concentrations are equal, and 2) the sum of the diffusible and nondiffusible anions on either side of the membrane is equal to the sum of the concentrations of diffusible and nondiffusible cations; the unequal distribution of diffusible ions thus produced creates a potential difference across the membrane (membrane potential). SYN: Gibbs-Donnan e..
dynamic e. SYN: e. (2) .
genetic e. the condition of a dynamic genetic system in which the several rates of change between all possible pairs of parts are such that the composition is invariant.
Gibbs-Donnan e. SYN: Donnan e..
Hardy-Weinberg e. that state in which the genetic structure of the population conforms to the prediction of the Hardy-Weinberg law; it is not a stable e., although for a large mating population it may be approximated. SYN: random mating e..
homeostatic e. homeostasis.
nitrogenous e. a condition in which the amount of nitrogen excreted from the body equals that taken in with the food; nutritive e. so far as protein is concerned.
nutritive e. condition in which there is a perfect balance between intake and excretion of nutritive material, so that there is no increase or loss in weight. SYN: physiologic e..
physiologic e. SYN: nutritive e..
radioactive e. a situation (not a true e.) in which a particular atom is being produced by the radioactive breakdown of a precursor while it is itself breaking down, the two breakdowns matching so that after a period of time the ratio of radioactivity of product and precursor is constant with time.
random mating e. SYN: Hardy-Weinberg e..
secular e. a type of radioactive e. in which the half-life of the precursor (parent) radioisotope is so much longer than that of the product (daughter) that the radioactivity of the daughter becomes equal to that of the parent with time.
stable e. e. in which, after every small perturbation, the original state will tend to be restored.
transient e. a type of radioactive e. in which the half-life of the parent radioisotope is longer than that of the daughter so that the ratio of activities of parent and daughter become constant as they decrease with time.
unstable e. e. in which the response to a small perturbation will tend to make the perturbation greater ( e.g., a logged feedback process of zero order).

equivalence, equivalency (e-kwiv′a-lens, -len-se)
1. The property of an element or radical of combining with or displacing, in definite and fixed proportion, another element or radical in a compound. 2. The point in a precipitin test at which antibody and antigen are present in optimal proportions. [L. aequus, equal, + valentia, strength (valence)]

equivalent (Eq, eq) (e-kwiv′a-lent)
1. Equal in any respect. 2. That which is equal in size, weight, force, or any other quality to something else. 3. Having the capability to counterbalance or neutralize each other. 4. Having equal valencies. 5. SYN: gram e.. [see equivalence]
combustion e. the heat value of a gram of carbohydrate or fat oxidized outside the body.
gold e. a unit of power of the protective colloids; the number of milligrams of protective colloid just sufficient to prevent the precipitation of 10 ml of a 0.0053–0.0058% gold solution by the action of 1 ml of a 10% sodium chloride solution. SYN: gold number.
gram e. 1. the weight in grams of an element that combines with or replaces 1 g of hydrogen; 2. the atomic or molecular weight in grams of an atom or group of atoms involved in a chemical reaction divided by the number of electrons donated, taken up, or shared by the atom or group of atoms in the course of that reaction; 3. the weight of a substance contained in 1 L of 1 n solution; a variant of (1). SYN: combining weight, e. weight, e. (5) .
Joule e. (J) the dynamic e. of heat; the amount of work converted to heat that will raise the temperature of 1 pound of water 1°F is 778 foot-pounds; in metric units, 1 calorie, which raises 1 g of water 1°C, equals 4.184 × 107 dyne-centimeters, or 4.184 J.
lethal e. 1. a combination of selective effects that on average have the same impact on the composition of the gene pool as one death; e.g., two carriers at 50% risk of dying would be the lethal e. of one carrier at 100% risk; 2. in the population genetics of recessive traits lethal e. is expressed as twice the sum of the expected number of deaths ascribable to the genetic load. 3. expression used of the genetic load of recessive genes in heterozygous state that if in homozygous state would cause death or carry a risk of death. The expected number of deaths from all such genes is expressed in lethal e..
metabolic e. (MET) the oxygen cost of energy expenditure measured at supine rest (1 MET = 3.5 ml O2 per kg of body weight per minute); multiples of MET are used to estimate the oxygen cost of activity, e.g., 3–5 METs for light work; more than 9 METs for heavy work.
nitrogen e. the nitrogen content of protein; used in calculating the protein breakdown in the body from the nitrogen excreted in the urine, 1 g of nitrogen considered as having originated in 6.25 g of protein catabolized.
starch e. the amount of oxygen consumed in the combustion of a given weight of fat as compared with that consumed in the combustion of an equal weight of starch; the figure is about 2.38, that for starch being taken as 1.
toxic e. the amount of toxin or other poison per kilogram of body weight necessary to kill an animal.

ER
Abbreviation for endoplasmic reticulum.

Er
Symbol for erbium.

eradication
Referring to disease, the termination of all transmission of infection by extermination of the infectious agent through surveillance and containment; global e. has been achieved for smallpox, regional e. for malaria and perhaps in some places for measles.

erection (e-rek′shun)
The condition of erectile tissue when filled with blood, which then becomes hard and unyielding; denoting especially this state of the penis. [L. erectio, fr. erigo, pp. erectus, to set up]

erector (erek′tor, -tor)
1. One who or that which raises or makes erect. 2. Denoting specifically certain muscles having such action. SYN: arrector. [Mod. L.]

ergocalciferol (er′go-kal-sif′er-ol)
Activated ergosterol, the vitamin D of plant origin; it arises from ultraviolet irradiation of ergosterol, which is cleaved at the 9,10 bond and develops a double bond between C-10 and C-19; used in prophylaxis and treatment of vitamin D deficiency. SYN: calciferol, ercalciol, viosterol, vitamin D2.